Pore structure characterization of different rank coals using gas adsorption and scanning electron microscopy

Eleven coal samples of different metamorphism are studied with regard to their pore structures. Both low-pressure nitrogen gas adsorption (LP-N2GA) and scanning electron microscopy (SEM) were performed. The use of these techniques allows us to gain clearer insight into the nature of the pore structure including the pore volume, specific area, pore size distribution (PSD) and pore shape. The LP-N2GA isotherms demonstrate strong differences in gas adsorption capacity between the coal samples studied, consistent with variability in specific surface area (SSA) of the samples. Pore geometry of coals with different metamorphism varies a lot, indicative of heterogeneity on coal surface, which was verified with SEM observation. Adsorption analysis revealed that mesopore size distributions are multi-modal, whereas, the micropore structure of the samples tested appears to be unimodal, with a major peak between 1.6 and 2.0 nm. The influence of coal rank on pore structure was also analyzed. The U-shape relationship between mesopore SSA and Vdaf is observed, demonstrating that the number of mesopores within the lower rank coals (Vdaf > 15%) decreases with increasing coal rank and the coalification mainly affects the mesopore structure. For the higher rank coals with Vdaf < 15%, as the coalification effect increases, the mesopore size diminishes and the number of micropores ascends. Smaller mesopores and micropores gradually become the dominant roles. This phenomenon is due to the effect of intensive compaction within the coal bulk. The combination of LP-N2GA and SEM techniques gives a better understanding of pore characteristics in coal. The research results will provide guidance for the gas control in coal mines.

[1]  C. S. Yust,et al.  Transmission electron microscope observations of porosity in coal , 1976 .

[2]  Victor Rudolph,et al.  Evaluation of coal structure and permeability with the aid of geophysical logging technology , 2009 .

[3]  Peter J. Crosdale,et al.  Role of coal type and rank on methane sorption characteristics of Bowen Basin, Australia coals , 1999 .

[4]  Philip L. Walker,et al.  Nature of the porosity in American coals , 1972 .

[5]  M. Lamberson,et al.  Coalbed Methane Characteristics of Gates Formation Coals, Northeastern British Columbia: Effect of Maceral Composition , 1993 .

[6]  Stuart Day,et al.  Methane capacities of Bowen Basin coals related to coal properties , 1997 .

[7]  Yanbin Yao,et al.  Pore structure and its impact on CH4 adsorption capacity and flow capability of bituminous and subbituminous coals from Northeast China , 2013 .

[8]  Harold D. Bale,et al.  Small-angle X-ray-scattering investigation of submicroscopic porosity with fractal properties , 1984 .

[9]  R. Flores.  Coal and Coalbed Gas , 2014 .

[10]  Yaodong Jiang,et al.  Pore Structure Characterization of Coal by Synchrotron Small-Angle X‑ray Scattering and Transmission Electron Microscopy , 2014 .

[11]  M. Yalçın,et al.  Pore volume and surface area of the Carboniferous coals from the Zonguldak basin (NW Turkey) and their variations with rank and maceral composition , 2001 .

[12]  T. L. Hill.  The structure and properties of porous materials , 1960 .

[13]  J. Faulon,et al.  Correlation between Microporosity and Fractal Dimension of Bituminous Coal Based on Computer-Generated Models , 1994 .

[14]  Maria Mastalerz,et al.  Porosity of Coal and Shale: Insights from Gas Adsorption and SANS/USANS Techniques , 2012 .

[15]  Celso Peres Fernandes,et al.  Characterization of pore systems in seal rocks using Nitrogen Gas Adsorption combined with Mercury Injection Capillary Pressure techniques , 2013 .

[16]  Quanlin Hou,et al.  Macromolecular and pore structures of Chinese tectonically deformed coal studied by atomic force microscopy , 2015 .

[17]  A. Cohen,et al.  Measuring surface properties and oxidation of coal macerals using the atomic force microscope , 2005 .

[18]  R. Flores.  Coalification, Gasification, and Gas Storage , 2014 .

[19]  Yanbin Yao,et al.  Fractal characterization of adsorption-pores of coals from North China: An investigation on CH4 adsorption capacity of coals , 2008 .

[20]  A. Neimark,et al.  Density functional theory model for calculating pore size distributions: pore structure of nanoporous catalysts , 1998 .

[21]  E. Cuerda-Correa,et al.  Determination of the fractal dimension of activated carbons: Two alternative methods , 2006 .

[22]  ZhongWen Ling.  Influence of specific pore area and pore volume of coal on adsorption capacity , 2002 .

[23]  Yuanping Cheng,et al.  Influence of Coalification on the Pore Characteristics of Middle–High Rank Coal , 2014 .

[24]  Mingjun Zou,et al.  Classifying Coal Pores and Estimating Reservoir Parameters by Nuclear Magnetic Resonance and Mercury Intrusion Porosimetry , 2013 .

[25]  Raymond C. Everson,et al.  Comparing the porosity and surface areas of coal as measured by gas adsorption, mercury intrusion and SAXS techniques , 2015 .

[26]  Junqian Li,et al.  Advanced characterization of pores and fractures in coals by nuclear magnetic resonance and X-ray computed tomography , 2010 .

[27]  Shaohua Wu,et al.  Morphological characterization of super fine pulverized coal particle. Part 2. AFM investigation of single coal particle , 2010 .

[28]  E. Teller,et al.  ADSORPTION OF GASES IN MULTIMOLECULAR LAYERS , 1938 .

[29]  Gyoung-Ja Lee,et al.  Characterisation of geometric and structural properties of pore surfaces of reactivated microporous carbons based upon image analysis and gas adsorption , 2006 .

[30]  K. Elewaut,et al.  Application of X-ray computed tomography for analyzing cleat spacing and cleat aperture in coal samples , 2006 .

[31]  H. Cohen,et al.  CO2 Adsorption Inside the Pore Structure of Different Rank Coals during Low Temperature Oxidation of Open Air Coal Stockpiles , 2011 .

[32]  Zhihua Liu,et al.  Fractal characterization of seepage-pores of coals from China: An investigation on permeability of coals , 2009, Comput. Geosci..

[33]  D. Avnir,et al.  Recommendations for the characterization of porous solids (Technical Report) , 1994 .

[34]  Song Xiao-xi,et al.  Pore structure in tectonically deformed coals by small angle X-ray scattering , 2014 .

[35]  Li Zi-we.  Characteristics of pore size distribution of coal and its impacts on gas adsorption , 2013 .

[36]  P. Hatcher,et al.  On the porous structure of coals: Evidence for an interconnected but constricted micropore system and implications for coalbed methane recovery , 1997 .

[37]  K. Kaneko,et al.  Multi-stage micropore filling mechanism of nitrogen on microporous and micrographitic carbons , 1990 .

[38]  R. Bustin,et al.  Geological controls on coalbed methane reservoir capacity and gas content , 1998 .

[39]  János Urai,et al.  BIB-SEM study of the pore space morphology in early mature Posidonia Shale from the Hils area, Germany , 2012 .

[40]  Christopher R. Clarkson,et al.  The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 1. Isotherms and pore volume distributions , 1999 .

[41]  C. Rodrigues,et al.  The measurement of coal porosity with different gases , 2002 .

[42]  Sevket Durucan,et al.  Gas Storage and Flow in Coalbed Reservoirs: Implementation of a Bidisperse Pore Model for Gas Diffusion in Coal Matrix , 2005 .

[43]  S. J. Gregg,et al.  Adsorption Surface Area and Porosity , 1967 .

[44]  Christopher R. Clarkson,et al.  Nanopore-Structure Analysis and Permeability Predictions for a Tight Gas Siltstone Reservoir by Use of Low-Pressure Adsorption and Mercury-Intrusion Techniques , 2012 .

[45]  R. Marc Bustin,et al.  The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs , 2009 .

[46]  M. Mastalerz,et al.  Application of SAXS and SANS in Evaluation of Porosity, Pore Size Distribution and Surface Area of Coal , 2004 .

[47]  Yanbin Yao,et al.  Comparison of low-field NMR and mercury intrusion porosimetry in characterizing pore size distributions of coals , 2012 .

[48]  P. Tarazona,et al.  Capillary condensation and adsorption in cylindrical and slit-like pores , 1986 .

[49]  Christopher R. Clarkson,et al.  Pore structure characterization of North American shale gas reservoirs using USANS/SANS, gas adsorption, and mercury intrusion , 2013 .

[50]  A. Neimark,et al.  Molecular Level Models for CO2 Sorption in Nanopores , 1999 .

[51]  Yujiro Ogawa,et al.  Mechanism of methane flow through sheared coals and its role on methane recovery , 2003 .